If you plan to use a micro-SD card as adoptable storage, be prepared to buy a properly rated card that can offer the fastest write speed. Attempting to save money by buying an inexpensive micro-SD card will have an out-sized negative impact to the performance of your phone or tablet. You will find that your apps may begin to stutter and your device will become slower or may seem to freeze as it spends more time waiting to write data to a slow SD card.
The adoptable storage feature allows an Android device (phone or tablet) to use an external micro-SD card as storage space for apps that otherwise would need to be saved on the device’s internal storage. This is especially useful for devices that come with a low amount of internal storage from the manufacturer.
I had purchased a Lenovo Tab4 10 Plus tablet (one of the very few tablets that enable the adoptable storage feature of Android) with 16GB of internal storage. At the time, I bought a U1-rated SanDisk Ultra micro-SD card as extra storage. The card was inserted into the tablet and formatted as adoptable storage. I was incredibly disappointed when the tablet began to suffer from frequent freezes and apps began to stutter. My first presumption was that I received a dud SD card, so I swapped the card for another U1-rated card from Silicon Power. Sadly, the experience with the Silicon Power SD card was no different from the SanDisk card. My initial presumption was that the read speed would be the determining factor. A faster read speed would have apps start up faster and load data quicker. Frustrated, I shelled out more money to buy a U3-rated Samsung EVO Plus SD-card. Using the U3-rated card brought the tablet performance back to a normal level.
Running some I/O benchmarks, I discovered that the write speed of the storage media is the most important factor for adoptable storage. The main difference between a U1 and U3 rated card is the write speed. The U3-rated card has a write-speed that is about double that of the U1-rated card, but two-thirds of the internal storage write speed.
| Storage | Read speed (MB/s) | Write speed (MB/s) |
| Internal storage | 177.8 | 48.8 |
| U1 SanDisk Ultra | 74.5 | 16.5 |
| U1 Silicon Power Elite | 67.8 | 18.2 |
| U3 Samsung EVO Plus | 79.5 | 31.6 |
Occasionally, the U3-rated card experiences stutters, but it is few and far between whereas the U1-rated card caused the tablet to be unusable. If you have an existing tablet with adoptable storage available, I would strongly recommend buying a U3-rated card. If you’re considering a new tablet, I would simply buy a tablet with enough internal storage for your needs. It is not worth the trouble and frustration dealing with adoptable storage and an external SD card when performance can vary so much. The extra money needed to buy a U3 SD card could be better used by purchasing a device with more internal storage instead.
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